Yea the personal calling via Onstar uses Verizon. The cool part is that now you can share minutes between your verizon phone and your Hummer, so you can use it as often as you want not have to buy minutes from Onstar at crazy prices.
I have the Iridium 9505 and have never had to use it. Just feel better when I'm out in nowheresville
I think I paid 1150.00 for it from a local vendor here, not a bad price for new.
Here is the link for share your Verizon minutes with your Onstar Personal Calling, if anyone is interested;
http://www.vzwshop.com/onstar/